Polarimetry Extraction and Sky Subtraction

The polarimetry recipes are designed to work with a Wollaston prism. This divides the signal into four partitions. These are ordinary and extraordinary beams (usually abbreviated to e and o beams) for the target and a region of nearby sky. Thus the raw data comprise four strips with an aspect ratio of about 6.

These partitions are normally separated by a mask, but the recipes do not depend on having the mask to extract the various regions say by detecting the edges. For each instrument the pixel limits of each region are fixed. The current target limits are 30% to 70% of the width of the long axis of each region to allow for some reasonable dithering of point sources, since there are usually only three jitter positions, while making mosaics with few pixels not derived from all contributing jittered frames. For extended sources these limits change to 10% to 90% to include as much object as possible with smaller dithers and alternating to blank-sky regions. Thus the limits define a section 40% or 80% of the width of the frame, roughly centred on the source. The limits on the sky regions are 1% to 99% of the frame width, mainly to avoid unreliable and pathological pixels at the detector's edge.
[<instrument>/_DEFINE_POL_REGIONS_]

The recipes extract the target regions into e- and o-beam frames. The modes (the means after clipping at 2, 3, 3 standard deviations) of the e- and o-beam sky regions are subtracted from their corresponding target beam, incorporating the uncertainty of each sky level in the corresponding target beam's variance array. For an extended-source observation, the sky levels are determined from the two corresponding regions for each beam in the following sky frame.
[_SUBTRACT_SKY_POL, _SUBTRACT_SKY_POL_EXTENDED_]